An automobile is provided with an alternator which is rotated by an engine and generates an electric power. During rotation of the engine, the electric power from the alternator is supplied to various electric loads, and, surplus power is stored in a battery.
In the alternator, a stator coil of a stator generates a three-phase alternating current (AC) when a rotor having a field coil is rotated. The AC power is converted to a DC by a three-phase bridge type rectifier having six diodes.
The voltage amplitude of the generated power is in proportion to the rotational speed of the rotor, and, the amplitude of a field current flowing in the field coil. Because a constant voltage must be kept for supplying the power to the load and for charging the battery, when the rotational speed becomes high, and the generated voltage goes over a predetermined value, a regulator controls the voltage by adjusting the field current.
The regulator adjusts the amount of the field current flowing in the field coil, based on the current supplied to the loads. A transistor type regulator controls the field current by changing a duty ratio, which is determined by ON/OFF of a power transistor. By adjusting the field current, the voltage generated by the alternator is controlled and over discharge or over charge of the battery is prevented.
The alternator increases the field current when the electric load increases, and decreased the field current when the electric load decreases. Thus, the torque required for rotating the alternator becomes large when the load increases, and the torque becomes small when the load decreases.
When the engine is operate in idling, if the electric load suddenly increases, then the rotational speed of the engine goes down because the torque required for rotating the alternator becomes large. Therefore, in a bad case, a driver feels an uncomfortable vibration of the automobile due to unstable rotation of the engine.
Because the engine generates a small torque when idling, if a slow idling speed of the engine is set for reducing fuel expense, then such discomfort occurs.
When the engine is idling, the driver is very sensitive to noise and fluctuations of the engine rotation. This is because his attention required is not for performing special operations and, there is little noise inside of the passenger compartment. In such a condition, when the engine speed drops by reason of an increase of the electric load, for example, operation of an air conditioner, the driver senses the drop in the engine speed.
Therefore, the driver may feel that the engine is going to stop. Besides, if the vibration increases, the discomfort to the driver also increases.
For preventing above-mentioned discomfort, the present invention provides a device for controlling changes in idling speed which can limit the drop of the rotation speed of the internal-combustion engine even if the electric load suddenly increases during idling.